Method for the preparation of aerogels

ABSTRACT

A method for the preparation of aerogels that involves the exchange of the liquid present in the aquagel with xenon and the subsequent extraction thereof.

The present invention relates to a process for the preparation ofaerogels including an exchange of the liquid present in the wet gel withxenon and a xenon recovery step.

Aerogels are one of the possible products of a sol-gel process.Aerogels, up to now, have found application mainly in the thermoacousticinsulation and in chemical catalysis, as well as intermediate materialsin production of glasses and glass ceramics; a new application,currently under study, is an insulation layer of very low dielectricconstant in the production of integrated circuits.

It is known that the sol-gel processes are chemical processes accordingto which a material is produced from a mixture of suitable precursors(called sol), such a material being typically a simple or mixed oxideeither as a bulk or as a thin layer onto a carrier.

Sol-gel processes are the subject of important published patentdocuments and are described, for example, in the following patents: U.S.Pat. No. 4,574,063, U.S. Pat. No. 4,680,048, U.S. Pat. No. 4,810,674,U.S. Pat. No. 4,961,767 and U.S. Pat. No. 5,207,814.

Water, alcohols and water/alcohol mixtures are usually employed assolvent/diluent for the starting solution, the precursors may be solublesalts of metals and/or of metalloids, for example nitrates, chloridesand acetates or preferably they may be compounds of the general formulaM(OR)_(n), where M is the metal or metalloid atom, O—R is an alcoholicradical (typically the alcoholic radical contains from one to fourcarbon atoms) and n is the valence of M. Among the most frequently usedprecursors in sol-gel processes are tetramethoxysilane (known as TMOS)with the formula Si(OCH₃)₄ and tetraethoxysilane (known as TEOS) withthe formula Si(OCH₂CH₃)₄.

The first phase of a sol-gel process is the precursor hydrolysis fromwater that may be present as solvent or be purposely added in the caseof alcoholic solutions, according to the reaction:M(—OR)_(n)+n H₂O→M(OH)_(n)+n ROH  (I)

This phase is generally helped by low pH values: typically between 0 and3, preferably between about 1 and 2.

The second phase in the sol-gel process is the condensation of theM(OH)_(n) species according to the following scheme:M(OH)_(n)+M(OH)_(n)→(OH)_(n−1)M-O-M(OH)_(n−1)+H₂O  (II)

This reaction, extended to all the M(OH)_(n) species originally presentin solution, leads to an inorganic oxidic polymer with an open structureinglobating within its porosity all the solvent originally present orgenerated during the hydrolysis. The inorganic, oxidic polymer soproduced is called gel.

To find practical application, the gel needs to be dried by carefulextraction of all liquid from its pores.

A possible method of drying a gel is by simple solvent evaporation; thedry gel so produced is known as a “Xerogel”. As it is known to theexperts in the field, the production of xerogels is extremely difficultbecause of the strong capillary forces produced by the solvent on thepore wall: during evaporation, that normally lead to the destruction ofthe gel.

An alternative method for producing dry gels is the supercritical (orhypercritical) extraction of the solvent. Dry gels produced by thistechnique are known as “Aerogels”.

During hypercritical drying in suitable autoclaves the liquid present inthe gel is subjected to temperature and pressure values that exceed thecritical values specific for that liquid. Under those conditions thewhole liquid volume passes from the liquid phase to that ofsupercritical fluid and the related capillary forces inside the poresdecrease from the initial value to a reduced value proper for thesupercritical fluid. This avoids a destructive phase associated withevaporations occurring during the preparation of xerogels due to thepresence of a meniscus inside the pores. The technique of hypercriticalextraction of a liquid from a gel is described, for example, in U.S.Pat. No. 4,432,956 and U.S. Pat. No. 5,395,805. The main problem withthis technique is that alcohols, normally present in the gel pores havecritical pressure Pc typically above 60-70 bars and criticaltemperatures Tc above 250° C. Such critical values require the use ofautoclaves of high resistance and relatively high cost. Moreover, if thegel product is in the form of film on support (for example in the caseof a dielectric insulating film on an integrated circuit), the criticaltemperatures of alcohols and esters might be too high and may not becompatible with the support or with other materials present on it.

A well known technique to overcome the problem is through the liquidexchange in the wet gel before hypercritical extraction, with a liquidof more favorable critical constants, particularly of a lower Tc. Forexample, it is possible to use hydrocarbons, as pentane and hexane, thathave critical temperature in the range of 200° C. Even in this case,however, the Tc value might be not compatible with all the applicationsassociated with aerogels; moreover, the exchange of a hydroalcoholicliquid with an hydrocarbon, because of the non-mixability of theseliquids would require an additional exchange with intermediate liquids,e.g., acetone, with a consequent lengthening of the processing time andthe added cost of recycling the organic solvents.

Still another possibility is to exchange the hydroalcoholic liquid withliquid CO₂, that has a very favorable Tc value (about 35° C.). Thisliquid, however, is not miscible with water and requires the use of anintermediate exchange liquid. Acetone, in this case, is not suitablebecause, if it is mixed with liquid CO₂, it prevents CO₂ from enteringhypercritical transition. It is possible, however, to use, as anintermediate exchange liquid, isoamylacetate; but the double exchange(acetone first, amylacetate second) requires an excessively longprocess-time for an industrial process and also undesirablesolvent-recycling costs.

Applicant has now found that it is possible to prepare the aerogelswithout the drawbacks associated with the prior art and, according to apreferred embodiment, through a hypercritical extraction step carriedout at moderate pressure and temperature values which, further, does notrequire long times associated with the preceding liquid exchange in thewet gel.

As a matter of fact, an object of the present invention is a method forthe preparation of aerogels comprising the exchange of the aquagelliquid phase with xenon, the extraction of xenon and the optionalrecovery thereof; particular advantages are achieved by carrying out theexchange with liquid xenon and the extraction thereof undersupercritical conditions.

The aquagel preparation can be made following one of the preparativeprocesses taught in the art; for example, by hydrolysis of a suitableprecursor. In this case, the process will involve a preliminary step ofhydrolysis/condensation starting from a suitable precursor.

An embodiment of the present invention is the preparation of theaerogels including:

-   -   a) hydrolysis/condensation starting from a precursor;    -   b) exchange of the liquid in the gel with xenon;    -   c) supercritical extraction of xenon;    -   d) optional xenon recovery.

The metallic precursor undergoing the hydrolysis reaction may be anysuitable compound known in the art. Therefore, use may be made ofsoluble salts, such as, nitrates, chlorides and acetate; and preferablyalcoxydes or alcoxyde mixtures having the general formula:X-Me- (OR)_(n−1)wherein Me is a metal of the 3^(rd), 4^(th) and 5^(th) Groups of theElement Periodic System; n is Me valence; X is R or OR, R being acidalkyl radical, linear or branched, having a carbon atom number up to 10.

The hydrolysis is carried out in the presence of a catalyst, preferablyan acid, and water may be the solvent, or it may be added to theprecursor alcoholic solution; the relevant conditions and procedure arereported in the known art such as, for instance, the one correspondingto U.S. Pat. No. 5,207,814 according to which the hydrolysis is carriedout at room temperature and the preferred acid catalysts are chlorideacid, nitric acid and acetic acid. Metal oxides, mainly silicon oxide,can be added to the prepared sol to modify the properties thereof,according to, for instance, U.S. Pat. No. 5,207,814.

The liquid present in the wet gel is exchanged with xenon havingcritical temperature, Tc=16,6° C. and critical pressure, Pc=58.4 bars,over very short times. Once the exchange is completed, xenon is easilyextracted without any use of autoclave suitable for high temperature andpressure.

Xenon is a gas at atmospheric pressure and temperature; it belong to theclass of the so called rare gases, and traditional is utilized indischarge lamps, in solar lamps, in arch lamps for the production ofU.V. radiations, to excite laser cavities, for ionization chambers, inthe bubble chambers for the detection of elementary particles.

For the purposes of the current invention, the xenon is maintainedliquid with pressure above 58.4 bars and temperature below 16.6° C.,preferably below 10° C. To favor interdiffusion processes in the liquidphase within the pore structure, xenon temperature should be not as low,usually not lower than 0° C.

Considering the high cost of xenon, the method of the invention ispreferably used with systems that provide the recovery of xenon at theend of the extraction process.

A possible scheme of this type of system is indicated in FIG. 1, towhich we here refer merely as an example excluding any restrictiveconsideration.

System A of FIG. 1 is an example of process of hypercritically drying anaquagel using exclusively xenon. The system includes a reservoir ofliquid xenon 1, connected through a line 2 to at least one, butpreferably several molds 3, (FIG. 1 shows a system of 3 molds)containing the original aquagels. The molds are connected with dischargelines 4, for the liquid exchanged in the pores (water and/or alcohol)and gaseous xenon at the end of the supercritical extraction process.All lines 4 are converging into one suitable collector 5, with propertemperature under which water and alcohol are solid while xenon is inthe liquid phase, for example at a temperature between about −30° C. and−40° C. Finally collector 5 is connected to a reservoir 6 for therecovered xenon, that in certain models could be the same reservoir 1.The system is supplied with open/closed valve, identified as V in FIG.1, that provide selective insulation of any component of the system. Inaddition, on the line 2 before each mold 3, there can be connectedflow-meters F, to regulate xenon flux in each mold; to each mold isconnected a pressure gage P, to control pressure in each mold.

More in general, liquid xenon at temperatures between 0° C. and 16° C.can be flown inside the aquagels to replace the liquid originallycontained so that a xenongel is obtained. The xenongel is then subjectedto hypercritical or supercritical drying at a temperature above 16.6° C.and pressure above 58.4 bars.

1. A process for the preparation of aerogels comprising, a) exchangingthe liquid phase of an aquagel with xenon; and b) extracting xenon fromthe xenon exchanged aquagel of step a); and c) optionally. recoveringxenon from step b).
 2. A process for the preparation of aerogelsaccording to claim 1, further comprising, prior to step a), forming anaquagel from a suitable precursor under conditions suitable forhydrolysis/condensation
 3. A process for the preparation of aerogelsaccording to claim 2, wherein the suitable precursor is an alkoxydehaving the formula:X-Me(OR)_(n−1) in which Me is a metal belonging to the 3^(rd), 4^(th)and 5^(th) Groups of the Element Periodic Table; n is integer andrepresents the valence of Me; X is either —OR or —R where —OR is analkoxyde group and —R is an organic radical, linear or branched, havingup to 10 carbon atoms.
 4. A process for the preparation of aerogelsaccording to claim 3 wherein the suitable precursor istetramethoxysilane or tetraethoxysilane.
 5. A process for thepreparation of aerogels according to claim 3 wherein hydrolysis is inpresence of an acid selected from hydrochloric, nitric or acetic acid.6. A process for the preparation of aerogels comprising a) forming anaquagel from a suitable precursor under suitable conditions forhydrolysis/condensation, b) exchanging the liquid phase of an aquagelwith liquid xenon; c) extracting xenon from the aquagel of step b) undersupercritical conditions; and d) optionally, recovering xenon from stepc).
 7. A process for the preparation of aerogels according to claim 6wherein the exchange is carried with liquefied xenon at temperaturebetween 0 and 16.6° C.
 8. A process for the preparation of aerogelsaccording to claim 6 wherein the super critical conditions include atemperature higher than 16.6° C.
 9. A process for the preparation ofaerogels according to claim 6 wherein the super critical conditionsinclude a pressure higher than 58.4 bar.
 10. A process for thepreparation of aerogels according to claim 1 comprises recovering xenonat the end of the extraction.